Aluminum anodizing method



3,252,875 ALUMINUM ANODIZING METHOD George Economy, New Kensington, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Filed Nov. 9, 1961, Ser. No. 151,181 7 Claims. (Cl. 20458) This invention relates to anodizing aluminum, and particularly to producing oxide coatings on an aluminum article by making the same anode in an electrolytic cell employing an aqueous acid electrolyte. As used herein, the word aluminum includes high purity aluminum, aluminum in various commercial grades, and aluminum base alloys.

Decorative and protective oxide coatings on aluminum have long been made by anodizing in electrolytes consisting of aqueous solutions of sulfuric acid, or oxalic acid, or sulfuric acid modified by minor additions of oxalic acid. Sulfuric acid electrolytes are usually employed at a temperature near room temperature, and must be cooled to maintain such temperature. The coatings, as so produced, are usually relatively clear, white or colorless, although on some alloys they may be tinted or colored by some constituents. Oxalic acid electrolytes are not as widely used, among other reasons because of the somewhat higher acid cost and the much higher anodizing voltages required with such electrolytes. They produce coatings that are relatively colorless, often with a yellow tinge. Mixed acid electrolytes, e.g. electrolytes containing 12 to 15 percent sulfuric acid and 1 to 4 precent oxalic acid, are also sometimes used to produce decorative and protective coatings similar to those produced in sulfuric acid.

The conventional decorative and protective anodic oxide coatings on aluminum made by use of the electrolytes above referred to are sometimes characterized as hard. However, in recent years the expression hard anodic coatings has been employed to identify extrahard and wear resistant anodic oxide coatings which are much more abrasion resistant than conventional coatings. They have been produced by anodizing in aqueous sulfuric acid electrolytes at extremely low temperatures, eg, 25 to 32 F., requiring much more cooling, with consequent increased equipment and operating expense. Such coatings, as so produced, in addition to being abrasion resistant, are colored in metallic gray, brownish gray and dark brown colors, which are attractive for architectural and other uses of aluminum. While hard anodic coatings have also been produced in electrolytes containing sulfuric acid and minor amounts of oxalic acid, operated at slightly higher temperatures, e.g. 50 F., such coatings are lighter in color than those produced at the extremely low temperatures in ordinary sulfuric acid electrolytes. producing coatings similar to those produced at extremely low temperatures are demanded.

It is a general object of this invention to provide, for use in an electrolytic cell for anodizing aluminum, a novel anodizing method for producing colored oxide coatings on aluminum. Another object is to provide such a method which is suitable for use under a wide range of practical operating conditions. A particular object is to provide such method for anodizing aluminum which requires no more than moderate cooling of the electrolyte. Another object is to produce abrasion resistant, lightfast, colored anodic oxide coatings on aluminum at moderate cost. A specific object of the invention is to produce uniform, abrasion-resistant, lightfast, colored anodic oxide coatings on aluminum, with moderate voltage and temperature requirements.

Consequently, more practical methods of United States Patent M Patented May 24, 1966 In accordance with the invention, it has been found that an electrolyte consisting essentially of oxalic acid, together with a much smaller amount of sulfate (as hereinafter more fully set forth) which is provided by at least one compound selected from the group consisting of sulfuric acid and water soluble sulfates and bisulfates, and water, may be advantageously used in an electrolytic cell for anodizing aluminum.

The oxalic acid may be employed in a very small amount, eg about 10 grams per liter (calculated as HO C-CO H-2H O), but preferably at least 20 grams per liter are employed, up to the limit of solubility at the operating temperature. In order to produce suitably colored coatings at moderate temperatures, most satisfactory results have been obtained when oxalic acid is present in an amount of about 45 to 70 grams per liter. Lower oxalic acid contents produce darker coatings but require somewhat higher anodizing voltages, and higher oxalic acid contents produce lighter coatings although lower anodizing voltages are required.

The sulfuric acid, or equivalent sulfate, or mixtures thereof, must be employed in a relatively small amount e.g. as little as 4 gram per liter (calculated as H may be employed. Preferably at least 1 grams per liter are employed, depending upon the amount of oxalic acid employed (as hereinafter set forth), since the voltage requirements for the same anodizing current density (with consequent electrical equipment, and electrolyte cooling cost factors to be considered) are reduced in such cases. For the production of suitably colored coatings at or near room temperature, e.g. when the oxalic acid content is about 45 to 70 grams per liter, most satisfactory results have been obtained when sulfuric acid or soluble sulfate is present in an amount of about 2 to 5 grams per liter. The sulfate content may be conveniently provided by sulfuric acid, an alkali metal or ammonium sulfate or bisulfate such as sodium sulfate or ammonium sulfate, a heavy metal sulfate or bisulfate such as ferric sulfate, an organic sulfate or bisulfate such as aniline sulfate, or any other water soluble sulfate or bisulfate. Double sulfates may also be used. Sulfuric acid is a preferred compound for introduction of sulfate, since it introduces no anions that may lead to ,plating out on the cathode or precipitation of insoluble compounds.

It has been discovered, as a particularly important aspect of the invention, that the ratio of oxalic acid (calculated as HO C-CO H-ZH O) to sulfate (calculated as H 80 for production of uniform, colored coatings at moderate voltages and temperatures, must exceed a certain critical value that is a function of the electrolyte temperature during anodizing. In particular, it has been found that the weight ratio (R) must be at least 9:1 at F., at least 11:1 at 75 F. and at least 13:1 at 60 F. Stating this relationship in a generalized way, the weight ratio of said oxalic acid to said sulfate :must be at least that expressed by the formula: R=2l- A T, where T is the electrolyte temperature in F. When these conditions are fulfilled, uniform lightfast colored, abrasion-resistant oxide coatings may be obtained under a wide variety of operating conditions, including moderate voltages and temperatures. It has been found that when this ratio (R) is only slightly lower, i.e. when sulfuric acid is present in even slightly larger amounts relative to the oxalic acid, the coatings are non-uniform; and at still lower values of this ratio (R), the coatings are substantially colorless. As a consequence of this discovery, it has been found desir able for any particular oxalic acid content chosen for the electrolyte, to provide a sulfate content near that at which the ratio (R) as above expressed is at or near its useable minimum. This reduces voltage and cooling re quirements, while permitting production of the coatings with a satisfactory color.

The following is a specific example of a preferred electrolyte suitable for use in accordance with the invention.

Electrolyte A Component: Grams per liter Oxalic (HO C-CO H'ZH O) 60 Sulfuric acid (H 50 4.5 Water Balance Grams per Liter Component Oxalic acid (CHaO-OH2C-2H2O) 20 90 Sulfuric acid (HZSOQ 1.5 2. 5 5 Water Balance in each case Other examples of electrolytes suitable for producing a uniform, colored coating, include the following:

Grams per Liter C omponent E F G H Oxalic acid (CH2O-OH2C-2H2O)- Sulfuric acid (H280 Water 7. a 2. Balance in each case Designation 4. satisfactory results, in producing colored coatings, especially with preferred electrolyte compositions such as Electrolytes A or C, above mentioned, and with a moderate amount of cooling being required, have been obtained with temperatures between about 55 and 90 F.

Current densities may extend over a wide range, without burnin of the coating, for practical purposes as low as 12 amperes per square foot or even less, and as high as 144 amperes per square foot or even more, depending on size, shape and composition of the aluminum article. Preferably, and without difficulties often associated with higher than conventional current density, the current density may be between 18 and 48 amperes per square foot. The current may be either AC. or DC, or a current of undulating characteristics, but D.C. is quite satisfactory.

Time of treatment depends upon current density and thickness of coating desired, as coating thickness is generally a function of anodizing current density and time. Colored, abrasion resistant coatings for outdoor service are often made in a thickness of 0.4 to 1 mil (0.0004 to 0.001 inch). However, coatings may be made in any substantial thickness desired, e.-g. 0.1 mil or greater.

Illustrative of particularly suitable operating conditions, Electrolyte A above mentioned has been operated at 60 or 75 F., at 24 or at 36 amperes per square 'foot. The voltages required start at about 40 to volts and generally end at about 40 to 85 volts, for time periods of 20 to 45 minutes, depending on the size of the load, the size of the tank, and composition of the aluminum being coated. As so operated, abrasion resistant, lightfast, colored oxide coatings on a variety of aluminum articles were produced. As examples of the colored coatings produced on various types of aluminum identified by commonly used designations and nominal :constitutents, it is noted that Electrolyte A, operated under the conditions just noted, for time willcient to produce coatings of the thicknesses indicated, produced the following results as to color:

Constituents 0.5-0.6 mil Mg, 0.25 Cr GB g 0.6 Mg, 0.25 Cr-.-

l LGB =Light; gray brown.

GB Gray brown. DB Dark brown. Bl=Black. MP G =Metallic pale gold. MLB =Nletallic light bronze. MMB =Metallic medium bronze.

D G =Dark gray.

2 Higher purity than 6063.

Electrolytes G and H, it is noted, should be operated at Color stability of the colored coatings was very high when elevated temperatures in order to retain the specified amounts of oxalic acid in solution; and Electrolytes F and H should be operated at sufficiently high temperatures for uniformity of coating in view of the indicated ratios (R) of such electrolytes.

As indicated above, electrolytes for use in accordance with the invention may be employed under a wide variety of operating conditions. A temperature at or somewhat above or below room temperature may generally be employed. Temperatures of at least about 55 F. and above may be employed to produce coatings of color similar to those produced at much lower temperatures in conventional sulfuric electrolytes. Higher temperatures produce somewhat lighter colored coatings than lower temperexposed in an Atlas Fade-Ometer for 1000 hours. For example, a 1.5 mil coating on 6063-T5 alloy, produced with a current density of 36 amperes per square foot and a treatment time of 30 minutes, in Electrolyte A at F., exhibited only a very small color change after exposure. That is, after an exposure of 1000 hours in the Fade- Ometer, the specimen exhibited a color difference E of only about 1.4. This result may be very favorably compared to results obtained with a similar coating produced in sulfuric acid electrolyte at 28 E, which exhibited a color difference E of about 8. Depending upon alloy and coating thickness, other coatings made in accordance with the invention exhibited color differences as low as 0.4 and not greater than 3.0. The color difference E is obatures, but reduce voltage and cooling requirements. Most 75 tained on a Color-Eye instrument by the ASTM D1495- 57T method of test. Lighter and darker shades of color were obtained with thinner and thicker coatings, respectively, as indicated.

As an example of the abrasion resistance of coatings produced under the above condition, 1 mil coatings on 6063-T5 aluminum exhibited abrasion resistance values in excess of 400 grams per mil, as obtained by the ASTM D658-44 method of test. These values are Well in excess of those of about 260 obtained on conventional coatings made in sulfuric acid electrolytes at room temperature, and are quite comparable to those for coatings made in sulfuric acid electrolytes at 28 F.

Coatings produced in accordance with the invention may be sealed, or dyed or pigmented and sealed, by conventional procedures. Thus, dyes or pigments may be used to modify the colors produced by the anodizing methods described herein. Etching or brightening treatments may be given the aluminum surfaces prior to anodizing, when desired.

What is claimed is:

1. A method of anodizing aluminum, comprising making an aluminum article anode in an electrolyte consisting essentially of at least grams per liter of oxalic acid (calculated as at least /4 gram per liter of sulfate (calculated as H 50 provided by at least one compound selected from the group consisting of sulfuric acid and water soluble sulfates and bisulfates, and water, the weight ratio (R) of said oxalic acid to said sulfate being at least that expressed by the formula: R=21? T, where T is the electrolyte temperature in F., while the electrolyte is maintained at a temperature between about 55 and 90 F.,

for a time of treatment sufficient to produce an anodic oxide coating of substantial thickness.

2. A method in accordance with claim 1 in which the current density is maintained between about 12 and 144 amperes per square foot.

3. A method in accordance with claim 1 in which the sulfate is provided by sulfuric acid.

4. A method of anodizing aluminum, comprising making an aluminum article anode in an electrolyte consisting essentially of at least about grams per liter of oxalic acid (calculated as HO C-CO H'2H O), at least about 1 /2 grams per liter of sulfate (calculated as H SO provided by at least one compound selected from the group consisting of sulfuric acid and water soluble sulfates and bisulfates, and water, the weight ratio (R) of said oxalic acid to said sulfate being at least that expressed by the formula: R=21 A T, where T is the electrolyte temperature in F.,

while the electrolyte is maintained at a temperature between about 55 and 90 F., and

6 the current density is maintained between about 18 and 48 amperes per square foot, for a time of treatment sufiicient to produce an anodic oxide coating at least 0.1 mil thick, 5 whereby a lightfast uniform colored anodic oxide coating is produced on the surface of the aluminum article. 5. A method in accordance with claim 4 in which the sulfate is provided by sulfuric acid.

6. A method of anodizing aluminum, comprising making an aluminum article anode in an electrolyte consisting essentially of about to 60 grams per liter of oxalic acid (calculated as HO CCO H2H O), about 2 to 5 grams per liter of sulfuric acid (H2804), and water, the weight ratio (R) of said oxalic acid to said sulfuric acid being at least that expressed by the formula: R=21-j4 T, where T is the electrolyte temperature in F., while the electrolyte is maintained at a temperature between about and 90 F., and the current density is maintained between about 18 and 48 amperes per square foot, for a time of treatment sutficient to produce an anodic oxide coating at least 0.4 mil thick, whereby a lightfast uniform colored anodic oxide coating is produced on the surface of the aluminum article. 7. A method of anodizing aluminum, comprising making an aluminum article anode in an electrolyte consisting essentially of about grams per liter of oxalic acid (calculated as about 4.5 grams per liter of sulfuric acid (H 80 and water, while the electrolyte is maintained at a temperature between about 60 and 75 F., and the current density is maintained between about 18 and 48 amperes per square foot, for a time of treatment sufficient to produce an anodic oxide coating at least 0.4 mil thick, whereby a lightfast uniform colored anodic oxide JOHN H. MACK, Primary Examiner.

MURRAY TILLMAN, Examiner.

60 R. HARDER, G. KAPLAN, Assistant Examiners. 

1. A METHOD OF ANODIZING ALUMINUM, COMPRISING MAKING AN ALUMINUM ARTICLE ANODE IN AN ELECTROLYTE CONSISTING ESSENTIALLY OF AT LEAST 10 GRAMS PER LITER OF OXALIC ACID (CALCULATED AS HO2C.CO2H.2H2O), AT LEAST 1/4 GRAM PERLITER OF SULFATE (CALCULATED AS H2SO4) PROVIDED BY AT LEAST ONE COMPOUND SELECTED FROM THE GROUP CONSISTING OF SULFURIC ACID AND WATER SOLUBLE SULFATES AND BISULFATES, AND WATER, THE WEIGHT RATION (R) OF SAID OXALIC ACID TO SAID SULFATE BEING AT LEAST THAT EXPRESSED BY THE FORMULA: R=21-2/15T, WHERE T IS THE ELECTROLYTE TEMPERATURE IN *F., WHILE THE ELECTROLYTE IS MAINTAINED AT A TEMPEATURE BETWEEN ABOUT 55 AND 90*F., FOR A TIME OF TREATMENT SUFFICIENT TO PRODUCE AN ANODIC OXIDE COATING OF SUBSTANTIAL THICKNESS. 